Improving the resolution of television pictures will be accompanied by a change in the size of the picture to approach the aspect ratios used for cine pictures. For a high-definition television system (HDTV), an aspect ratio of 5/3 is recommended in Japan. Other proposals even specify still higher values (5.3/3 or 5.5/3, for example).
In Europe, where the number of television channels is very limited, (for example, for transmission by satellite, only five channels per nation in the 12 GHz band), because of the large number of countries and of different languages, it is very important to ensure compatibility between the new intended aspect ratio and the existing 4/3 aspect ratio which will anyway ensure the majority of programs till the end of the century. Compatibility is here understood to mean that the first generation receivers can receive of course with the 4/3 aspect ratio as well as, the new transmissions with a 5/3 aspect ratio.
T. J. Long of the IBA has proposed such a system in a paper presented to the Montreux Symposium 1983 ("Why HDTV!", T. J. Long, Independent Broadcasting Authority, United Kingdom, pages 27-47) in the case of an initial standard of the C-MAC type. To arrive at the 5/3 aspect ratio without loss in picture height, he recommends two means:
(a) reducing the duration of the data burst to 20 Mb/s at the beginning of each line, from 10 .mu.s to approximately 3 .mu.s which results in only one stereo program instead of 8 sound channels, this reduction rendering it possible to bring the number of samples of the chrominance signal C and of the luminance signal Y from 350 to 400 and from 700 to 800, respectively, which results in a 4.6/3 aspect ratio;
(b) to obtain the aspect ratio 5/3, transmitting 80 additional samples Y and 40 additional samples C corresponding to both extreme left and right edges of the picture during line periods which are free during the field retrace periods.
The compatibility of this system will be ensured when first generation MAC receivers which operate in the 4/3 aspect ratio are used provided that, in these receivers, there is a possibility of changing the duration of the data burst, the position of the read period of the alignment level which provides the reference of the average grey level, and the beginning and the end of the periods of storing the samples C and Y in the memory. In the C-MAC system defined in the second revised version of the standard SPB 284 (June 1983) of the European Broadcasting Union (EBU), all these information components are contained in line 625, and consequently there are no inherent difficulties to provide this compatibility. It is, however, possible to utilize simpler solutions, for example reading some bits only which will indicate that a change to the 5/3 aspect ratio is being made, it being possible for these few bits to be situated either in line 625 (in SDF SCR, example, S D F S C R of the Static Data Frame Service Configuration Reference, being a word, defined by the standard which itself defines the configuration of the services or in MVSCG, M V S C G, of the Multiplex and Video Scrambling Control Group, which is also a word, defined by the standard which describes the physical organization of the signal) or in the special path of the sound/data multiplex conveyed by packets having the address zero (in VCONF, for example V C O N F of the Video Configuration, being a word, defined by the standard describing the configuration of the video signal). Knowing this, one might enter the boundaries of the different intervals for data, alignment level, chrominance and luminance into a read-only memory (ROM). It would furthermore be possible to provide this ROM in such a way that it is interchangeable such that it is not necessary to have all the characteristics of the second generation system specified already, at the 5/3 aspect ratio, before starting the manufacture of the first generation receivers, with the 4/3 aspect ratio. It is, however, possible to express two types of criticism as regards the solution proposed by the IBA:
(a) reducing the duration of the data burst reduces the capacity of the digital path by a ratio of 4, which is already much in a C-MAC system using an instantaneous 20 Mb/s rate, since this leaves only one stereo channel, but it becomes unacceptable for a D2-MAC system utilizing an instantaneous rate of 10 Mb/s, as this would leave only one single mono channel;
(b) the transmission of the additional samples during line periods necessitates the use of large-capacity fast memories and poses inter alia the problem that transitions become visible at the left and right of the picture, which transitions may appear from the moment the transmission channel introduces a signal distortion due to, for example, limitation of the passband or due to echo phenomena.